I agree. Your boot files are on the XP partition in all probability. Check
in Administrative tools/Computer management/Disk management. The partition
labeled system is where your boot files are and they must be copied to the
other partition with Vista on it before even attempting to get rid of XP.
You can play with it without cutting off your options if you are comfortable
with the command level operations of Command Prompt.
You also could leave the XP partition, delete the XP files leaving the boot
files for Vista, then with Vista compatible partitioning software shrink the
XP partition and add the freespace to the Vista partition then use
VistaBootPro or EasyBCD to eliminate the XP boot entry.
Your cleanest, safest way is as Jawade says, reinstall, and you can use
Windows Easy Transfer to make it easier. I would have backups of everything
you value, as WET hasn't been the most reliable product and I would not
depend on that as the only place my valuable data is.